1. Field of the Invention
The present invention relates generally to vehicle axles and more specifically to trailer axles.
2. Background
A vehicle's axle supports the weight of the vehicle and load and also provides a shaft upon which the wheels revolve. Truck rigs carrying heavy loads over long distances provide substantial forces to the axles. The allowable axle loads are restricted by law, but typically tandem truck trailer axles are expected to carry loads of up to 34,000 pounds per tandem. In fact, the camber of the wheels on these loaded axles can be altered by these forces such that the wheels are closer together at the top then at the bottom. This condition is known as negative camber.
Most trailer axles are manufactured with no intentional camber but with an acceptance that when in use, there will be some degree of camber deflection attributable to load. On a new trailer axle, manufacturers typically accept a camber alignment tolerance of about one quarter of a degree, positive or negative. This tolerance applies to the axle beam itself with no load applied and with no consideration for additional camber deflection that may be experienced by components of the hub and the wheel. On a typical truck semi-trailer, even with the minimal load applied by the weight of the empty trailer, each axle end may have a load of approximately 2000 pounds. This load is sufficient to cause the wheels to exhibit a negative camber orientation.
A fully loaded trailer may support a load of about 8500 pounds to each axle end and typically causes as much as one half degree or more of negative camber at the tire. This negative camber indicates that the contact surface of the tires is not parallel to the road surface. Consequently, the tread may wear unevenly, with the inner shoulder wearing most rapidly. When the truck is driven with a reduced load, the inner shoulder may not make firm contact with the ground, which allows slipping.
One solution to the problem above is to remove the axle and deflect it mechanically, or by other means, in the opposite direction. An example of one known technique for effecting a camber change to an axle is by applying heat to the axle to accomplish a plastic deformation. However, manufacturers frequently disapprove of such practices because the variables of the necessary plastic deformation are difficult to control and often explicitly void the warranty. Furthermore, this practice is time consuming and it is difficult to achieve the precise camber required without iterative trials.